1. Field of the Invention
The present invention relates to a method of forming a photoresist pattern and, more particularly, to such method which uses a chemically amplified photoresist preferably applied to a method of manufacturing a semiconductor device such as a DRAM.
2. Description of the Prior Art
In the manufacture of semiconductor devices represented by LSIs (Large-Scale Integrated circuits), a photolithography technique is indispensable to patterning various types of thin films including an insulating film, e.g., a silicon oxide film or silicon nitride film formed on a semiconductor substrate, and a conductive film such as an aluminum alloy film or copper alloy film, into a desired shape.
In the photolithograpy technique, conventionally, a photoresist photosensitive to ultraviolet radiation is applied to a thin film to form a photoresist film, and after that ultraviolet radiation is irradiated (exposed) to the photoresist film through a mask pattern to convert a region irradiated with ultraviolet radiation to a solubilized (positive) region or to convert a region not irradiated with ultraviolet radiation to a solubilized (negative) region. Subsequently, the photoresist film is developed, and the solubilized region is partly removed with a solvent to form a resist pattern. Then, the thin film is selectively etched using the resist pattern as a mask to pattern the thin film.
As the material of the photoresist described above, a positive novolac-based photoresist is generally, conventionally used. Since the positive photoresist has a higher resolution than that of a negative photoresist, most of photoresists of the type described above are of a positive type. As an exposure light source for the photoresist, a high pressure mercury lamp is used, and ultraviolet radiation, e.g., a g line (with a wavelength of substantially 436 nm) and an i line (with a wavelength of substantially 365 nm), generated by the high pressure mercury lamp is utilized.
As the integration degree of LSIs increases, a photolithography technique capable of a finer process is required, and accordingly the exposure light source for the photoresist tends to use ultraviolet radiation with a shorter wavelength with which a high resolution can be obtained. As a result, a photolithography technique using an excimer laser which generates far-ultraviolet radiation with a shorter wavelength than that of the i line described above as the light source (for example, when KrF (krypton fluoride) is used as the laser medium, the wavelength is substantially 248 nm) has been realized.
When the novolac-based photoresist described above is exposed by the KrF excimer laser light source described above, as the novolac-based photoresist absorbs a large quantity of light, a good resist pattern is difficult to obtain. Hence, as a photoresist which can realize a photolithograpy technique capable of a finer process in combination with a light source that can obtain far-ultraviolet radiation as described above, for example, a chemically amplified photoresist as described in Japanese Examined Patent Publication No. 2-27660 has been proposed.
A chemically amplified photoresist is a photoresist to which acid catalyst reaction is applied, as described in the above reference, and is roughly comprised of a base resin, e.g., polyhydroxystyrene (PHS), which becomes insoluble to alkali when protection groups are coupled to its predetermined portion and soluble to alkali when protection groups are free from its predetermined portion, an optical acid generating agent which generates hydrogen ions (acid) upon irradiation with light, a very small amount of additive for performance adjustment, and an organic solvent for spinner coating.
This chemically amplified photoresist is applied to a semiconductor substrate, dried, and solidified, and far-ultraviolet radiation emitted from an excimer laser as a light source irradiates a photoresist film on the obtained semiconductor substrate. Then, the optical acid generating agent generates hydrogen ions serving as the trigger species of chemical amplification. The hydrogen ions substitute the protection groups coupled to the base resin during a post exposure bake (PEB) process performed after exposure, so that the protection groups are eliminated. The photoresist which is insoluble to alkali is thus changed to be soluble to alkali. Also, since hydrogen ions are generated subsidiarily during this process, a chain reaction for eliminating the protection groups from the base resin progresses. This reaction is called an acid catalyst sensitization reaction. This acid catalyst sensitization reaction increases the solubility selectivity of the photoresist, so that highly photosensitive characteristics can be realized. Therefore, after exposure, if this photoresist is developed with an alkali developer, a desired very fine resist pattern can be obtained.
FIG. 1 is a process view showing a conventional method of forming a photoresist pattern using the chemically amplified photoresist described above in the order of steps. This photoresist pattern forming method will be described in the order of steps with reference to FIG. 1.
As shown in step A of FIG. 1, the surface of a semiconductor substrate having a desired thin film, where a photoresist pattern is to be formed, is subjected to a hydrophobic process, so that the adhesion of the photoresist is increased. As shown in step B, for example, a positive photoresist made of a chemically sensitizable type photoresist, containing a base resin formed of polymer compounds having a terpolymer structure as shown in formula (1) and an optical acid generating agent with a structure as shown in formula (2), is applied to the semiconductor substrate in accordance with spin coating, thereby forming a photoresist film. 
Subsequently, as shown in step C of FIG. 1, the photoresist film is prebaked to remove the solvent from it. The semiconductor substrate is cooled to room temperature, as shown in step D, and the photoresist film is irradiated with far-ultraviolet radiation from, e.g., a KrF excimer laser, through a mask pattern drawn with a desired pattern, to expose it, as shown in step E. The photoresist film is then subjected to a PEB (Post Exposure Bake) process to promote elimination reaction (acid catalyst sensitization reaction) of eliminating the protection groups from the photoresist film, as shown in step F.
The semiconductor substrate is then cooled again to room temperature, as shown in step G of FIG. 1, and the photoresist film is developed with an alkali developer to form a resist pattern, as shown in step H. The photoresist film that forms the resist pattern is post-baked to remove the water content produced by development, as shown in step I.
Subsequently, the thin film on the semiconductor substrate is selectively etched by using the resist pattern described above as a mask to pattern the thin film.
When a photoresist pattern is to be formed by using a chemically amplified photoresist, in recent semiconductor devices, a focal depth S must be set to about 0.7 xcexcm or more so that the manufacturing yield is increased. The focal depth S depends on a blocking level C of the employed photoresist. The blocking level C is determined by a ratio of numbers x, y, and z of repeating units in the formula (1) described above, and is expressed as:
Blocking level C=((x+y)/(x+y+z))xc3x97100(%)
FIG. 4 is a graph for explaining the relationship between the focal depth S (axis of ordinate) and the blocking level C (axis of abscissa). In the example shown in FIG. 4, a photoresist pattern having a contact hole with a diameter of 0.2 xcexcm is formed. As is apparent from FIG. 4, the focal depth S increases linearly proportionally where the blocking level C falls within a range of about 42%, but tends to saturate gradually when the blocking level C exceeds about 42%. In order to obtain the focal depth S of 0.7 xcexcm or more as described above, the blocking level C must be increased to about 40% or more.
In the conventional photoresist pattern forming method described above, when a chemically amplified photoresist with an increased blocking level is used, development defects increase.
More specifically, as described above, when a photoresist pattern is formed by performing exposure and development by using a chemically amplified photoresist with the increased blocking level C of about 42% or more in order that the focal depth S is set to about 0.7 xcexcm or more, many defects are formed during development. FIG. 2 shows these defects by exhibiting a relationship between the number N (axis of ordinate) of development defects and the blocking level C. As is apparent from FIG. 2, the number N of development defects increases sharply as the blocking level C exceeds about 38%, until reaching as many as almost 1,000. If these many development defects N exist in a photoresist pattern in this manner, this pattern is defective.
Therefore, conventionally, the focal depth S of about 0.45 xcexcm or less corresponding to the blocking level C of about 38% or less can only be obtained, as is apparent from FIG. 4, and a decrease in manufacturing yield cannot be avoided. FIG. 3 shows this conventional drawback in more detail by exhibiting a relationship between a resist pattern size (axis of ordinate) and a focal position (axis of abscissa), and explains a case wherein a contact hole with a diameter of 0.2 xcexcm is to be formed. A range M defined by broken lines L1 and L2 shows the allowable range of the target contact hole. FIG. 3 shows that the focal depth S of 0.3 xcexcm can only be obtained centered on a focal position of 0 xcexcm.
Conventionally, when the blocking level is increased, development defects increase. This is supposed to be due to the following reason. Since the chemically amplified photoresist used in the conventional photoresist pattern forming method is highly hydrophobic, a component solved by the development may attach to the photoresist film again to cause a development defect.
The present invention has been made in view of the above situation, and has as its object to provide a method of forming a photoresist pattern which can suppress development defects even if the blocking level of a chemically amplified photoresist is increased.
In order to achieve the above problem, according to the present invention of claim 1, there is provided a method of forming a photoresist pattern which comrises the steps of coating a chemically amplified photoresist containing an optical acid generating agent onto a substrate to form a photoresist film, subjecting said photoresist film formed in said coating step to predetermined patterning exposure, performing a PEB process (post-exposure bake process) on said photoresist film to promote acid catalyst sensitization reaction within an exposed region of a whole region of said photoresist film, and developing said photoresist film which has been subjected to the PEB process step, to obtain a photoresist pattern with a desired shape, characterized by a step of applying a defect preventive agent containing a hydrophobic group and a hydrophilic group to said photoresist film after said photoresist film is formed by coating but before developed.
The invention according to claim 2 relates to the photoresist pattern forming method according to claim 1. In claim 2, the defect preventive agent is applied before the exposure step.
The invention according to claim 3 relates to the photoresist pattern forming method according to claim 1. In claim 3, the defect preventive agent is applied after the exposure step.
In the invention according to claim 4, the defect preventive agent is applied to the photoresist film after a cooling step.
The invention according to claim 5 relates to the photoresist pattern forming method according to claim 1, 2, or 3. In claim 5, a surfactant is used as the defect preventive agent.
The invention according to claim 6 relates to the photoresist pattern forming method according to claim 5. In claim 5, the surfactant contains a carbon fluoride group or dimethylpolysiloxane group.
The invention according to claim 7 relates to the photoresist pattern forming method according to claim 5. In claim 7, the surfactant is applied by spin coating.
The invention according to claim 8 relates to the photoresist pattern forming method according to any one of claims 1 to 7. In claim 8, the chemical amplificaion type photoresist has a blocking level of 38% to 100%.
The invention according to claim 9 relates to the photoresist pattern forming method according to claim 8. In claim 9, the chemically amplified photoresist contains a polymer compound having a terpolymer structure.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.